Hi Ivan -- at the end of this, once you've worked your way through every battery technology known to humankind, will you be presenting an overall comparison matrix showing the various technologies in relation to each other?

As Mark Twain might have said, Everyone would like to get rid of the lead-acid battery, but nobody does!

I have been working on a battery backup unit design for industrial power supplies (24VDC) for 12 years or so. There is an old English joke-

Q: How do you start a pudding race?

A: Say go (ask Max to explain)

The project has not gone anywhere because management has never allocated the resources, but the need remains. Part of the issue of the design is that you need to charge a nominal 24VDC battery with a 24VDC power supply. There are several approaches to this, buit our current proposal is a buck/boost switchmode power supply. Boost for when the battery exceeds 24V and buck for the other case.

Anyway one of the considerations is that in order to test the battery, I am told you need to drain the battery through a load. The problem is that once this test is done, the battery needs to be recharged- what happens if backup is needed during the charge cycle? Do you concur with this concept that lead-acid batteries be regularily load-tested?

" ...in order to test the battery, I am told you need to drain the battery through a load. The problem is that once this test is done, the battery needs to be recharged- what happens if backup is needed during the charge cycle? Do you concur with this concept that lead-acid batteries be regularily load-tested?..."

Yes, to test it, you must do a load test (but the load you use is your own design). Lead-acid is notorious for looking fine at the very end of its service life; that is, until you need to get some juice from it. It is so important, that alarm systems and uninterruptable power supplies (UPS) will do a load test periodically, to verify that the battery is viable. In many backup systems by APS for example, the load test is substantial, and the unit shuts down the output for a few seconds to complete the test, everytime the unit is plugged into AC power, and everytime the power-on button is pressed. In alarm systems, and some emergency-EXIT signs, the test is scheduled. For these systems, the output is always live, and only done while there is AC power, to have continuous/uninterrupted service. Float charging replenishes the load-test energy. For those systems that are always completely battery powered, the load test is very light, and lasts an extremely short time, so that the battery will not be overly drained. This can be designed to keep power to the load duriing the test, also. Designers will select a battery that is a higher capacity, to just account for the extra energy that load-testing drains during its service life. Once a very specific battery is chosen, its characteristics are well known as it ages. Therefore, it is possible to do a light load for a very short time interval, and know the battery is ok -- especially if the device has a real-time calender to tell you how old the battery is. Some smart batteries include this function.

Recent Brad Albing blogged about a new TI chip that acts as a gas guage for lead-acid batteries. It looked good to an innocent like myself, and I was wondering if you thought the product had any merit. Looking at it, it does seem to need to apply a load to guage the remaining charge, but so much of the topic is new to me that I would like an expert's opinion.

"Recent Brad Albing blogged about a new TI chip that acts as a gas guage for lead-acid batteries. It looked good to an innocent like myself, and I was wondering if you thought the product had any merit. Looking at it, it does seem to need to apply a load to guage the remaining charge, but so much of the topic is new to me that I would like an expert's opinion."

The chip is very nice. It turns on a MOSFET periodically, to measure the battery terminal voltage. When it does this, it draws a small extra current, which is compared to [their proprietary piece]; doing this allows the internal resistance to be measured (see my previous blog on this). See my other replies on this topic, about the need to do load tests on lead-acid.

"Can you explain how the technologies for this overcome the gassing that is usual with the wet-cell types?"

looking at the chemical reaction, the GEL is esentially absorbing the Hydrogen, before it can accumulate into a bubble. But in addition, the chemistry/charge-voltage will not produce enough at a rate high enough, unless the cells are overcharged. You will see a warning put out by many manufacturers about popping the valve and explosion hazard, for overcharging due to Hydrogen outgassing.

"And how do the so-called Deep-cycle batteries avoid the sulphation that happens with ordinary batteries if they are deep-discharged?"

Some manufacturers use an additive, that disolves elemental sulfur, keeping it in solution. Others modify the plates to alter the ion density, to slow the movement of the heavier lead-sulfur crystals so it cannot stick. Another clever approach (specialty battery) is a mechanical structure that monitors the specific gravity and adds a buffer from a reservoir, which reverses at full charge. Deep cycle batteries need to be recharged fully, and done soon after the deep discharge is over. Self discharge is the enemy for sulfation. Folks don't realize the trade-offs that deep-cycle causes, such as lower cycle life, lower pulse-current ratings, etc.

Thanks Ivan. Could you also comment on the many designs - both commercial and presented as DIY projects - that claim to recover capacty in used batteries? As far as I can tell they seem to work by a very short high overcharge, I think to get rid of sulphation, but I may be wrong. Are any of these worth anything?

will be following their work. Reading over the Grants is particularly enlightening. BIC seems similar to the program the government just launched to drive innovation... Wonder if there is/will be a connection.

What is the recommended top charge voltage for gel, flooded, and agm batteries?

I have heard that temperature compensation helps prevent overcharging, particularly when a battery undergoes wide temperature changes while being charged. How much should the charging voltage be adjusted pr. °C for each cell?

"...Float-charge voltage per cell: 2.23 (gel), 2.32 (flooded), 2.25(AGM).At what temperature are those voltages valid?What is the recommended top charge voltage for gel, flooded, and agm batteries?I have heard that temperature compensation helps prevent overcharging, particularly when a battery undergoes wide temperature changes while being charged. How much should the charging voltage be adjusted pr. °C for each cell?"

In keeping up to date, I ran across this piece of industry news from www.powerpulse.net:

To address the impact of Partial State of Charge (PSOC) on cycling batteries in renewable energy (RE), inverter backup and telecom applications, Trojan Battery Co. today announced the addition of Smart Carbon™ as a standard feature to its Industrial and Premium flooded battery lines. Smart Carbon is a proprietary Trojan formula which provides improved performance when the batteries operate in PSOC, enhancing overall battery life in off-grid and unstable grid applications where the batteries are under charged on a regular basis. Along with increased life in a partial state of charge, Trojan's Smart Carbon proprietary formula also provides improved charge acceptance and faster recharge in PSOC applications. http://www.powerpulse.net/story.php?storyID=29173

I have no affiliation to the news journal or to any battery companies. I just pass along possibly useful info. Some of you have been wondering about additives, so here is yet another.